The present invention relates to optical micro-electro-mechanical devices (xe2x80x9cMEMs devicesxe2x80x9d) and, in particular, to magnetically packaged MEMs devices. The inventive MEMs devices are particularly useful as movable mirror devices for beam steering in optical communication systems.
Optical MEMs devices are of considerable importance in optical communication systems. In one important application, a MEMs device provides a two-dimensional array of movable components, such as mirrors, to receive signals from optical input fibers. Each movable component in the array can be electrically moved in relation to the received optical input.
A typical MEMs mirror device comprises an array of metal-coated silicon mirrors, each mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal and define the mirror""s axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthogonal to that of the mirror. A light beam can therefore be steered in any direction.
Electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode and between the gimbal and an electrode control the orientation of the mirror. Alternatively, in slightly modified arrangements, an electrical signal can control the position of the mirror magnetically or piezoelectrically.
FIGS. 1(a) and 1(b) illustrate conventional optical MEMs devices and their application. FIG. 1(a) shows a typical prior art optical MEMs mirror structure. The device comprises a mirror 10 coupled to a gimbal 11 on a polysilicon frame 12. The components are fabricated on a substrate 9 by micromachining processes such as multilayer deposition and selective etching. After etching, mirror assembly (10, 11, 12) is raised above the substrate 9 by upward bending lift arms 13 during a release process. The mirror 10 in this example is double-gimbal cantilevered and attached onto the frame structure 12 by springs 14. The mirrors can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation with electrical voltage or current supplied as to electrodes 16 from outside. The light-reflecting surface of mirror 10 comprises a metal-coated polysilicon membrane, which is typically of circular shape. The metal layers 15 are deposited by known thin film deposition methods such as evaporation, sputtering, electrochemical deposition, or chemical vapor deposition.
FIG. 1(b) schematically illustrates an important application of optical MEMs mirrors as controllable mirror arrays for optical signal routing. The cross connect system shown here comprises optical input fibers 120, optical output fibers 121 and an array of MEMs mirrors 122 on a substrate 123. The optical signals from the input fibers 120 are incident on aligned mirrors 122. The mirrors 122, with the aid of a fixed auxiliary mirror 124 and appropriate imaging lenses 125, are electrically controlled to reflect the incident optical signals to respective output fibers 121. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMs mirror arrays are used to perform the cross connect function.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes (not shown) beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer alter the electric field, which affects the degree of mirror tilting. This in turn alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
An array of such MEMs mirrors is essentially composed of two layers: a component layer comprising the array of mirror elements movably coupled to a surrounding frame and an actuator layer comprising the electrodes and conductive paths needed for electrical control of the mirrors. One approach to fabricating the array is to fabricate the actuator layer and the component layer as successive layers on the same workpiece and then to lift up the component layer above the actuator layer using vertical thermal actuators or stresses in thin films. This lift-up process is described in U.S. patent application Ser. No. 09/415,178 filed by V. A. Aksyuk et al. on Nov. 8, 1999 and assigned to applicant""s assignee.
An alternative approach is to fabricate the component layer on one substrate, the actuator layer on a separate substrate and then to assemble the mating parts with accurate alignment and spacing. The two-part assembly process is described in U.S. Pat. No. 5,629,790 issued to Neukermans et al. on May 13, 1997 and in U.S. patent application Ser. No. 09/559,216 filed by Greywall on Apr. 26, 2000, both of which are incorporated herein by reference. This two-part assembly provides a more robust structure, greater component packing density and permits larger component sizes and rotation angles as well as scalability to larger arrays.
In the two-part assembly process, the component layer and the actuator layer are conventionally bonded together for mechanical sturdiness and long-term reliability. Neukermans et al. and Greywall suggest anodic bonding, solder glass bonding, and epoxy bonding. U.S. patent application Ser. No. 09/705,350, filed by D. W. Carr et al. on Nov. 3, 2000, patented U.S. Pat. No. 6,442,307 describes solder bonding of the layers. The gap spacing between the component layer and the actuator layer determines the electric field for the given magnitude of applied voltage (or the magnetic field for the given electrical current level). Therefore, an accurate and reliable establishment of the gap spacing during the assembly and bonding of the two layers, as well as the dimensional stability of the gap during device handling, shipping and operation are important. The accurate lateral alignment of the mating parts of the components and electrodes is also desirable for reliable operation.
To retain the accurate lateral alignment of the component layer and the actuator layer once the alignment is achieved, often requires high temperature bonding processes such as soldering at xcx9c100-300xc2x0 C., epoxy curing at xcx9c100-200xc2x0 C., polyimide curing at xcx9c250-400xc2x0 C., glass frit bonding (sometimes called glass solder bonding) at 400-700xc2x0 C., or anodic bonding at 400-900xc2x0 C. But the exposure of the MEMs components to temperatures even as low as xcx9c150xc2x0 C. can cause undesirable distortion or curvature. If the components are mirrors, heat can also cause metallurgical reactions at the interfaces between the mirror metallization and the silicon substrate with consequent contamination of the mirror metal, creep and dimensional changes, formation of brittle intermetallic compounds, and long-term reliability problems. The bowing or curving of the mirrors generally results in non-focused or non-parallel light reflection and loss of optical signal. Accordingly, there is a need for an assembly process that can be carried out at ambient temperature without having to expose the MEMS device to high temperature.
In accordance with the invention, the component layer, the spacer and the actuator layer of a MEMs device are assembled at ambient temperature and held together in lateral alignment by upper and lower magnets. Such ambient temperature magnetic packaging greatly minimizes the undesirable exposure of the sensitive MEMs components to high temperatures. The resulting MEMs device exhibits the high dimensional accuracy and stability. In a preferred embodiment, the component layer comprises a layer of movable mirrors and a spacer aerodynamically and electrostatically isolates each mirror, minimizing cross-talk between adjacent mirrors.